The use of gas turbine power generators has become a very attractive alternative for satisfying the increasing power needs of the United States and other nations. For example, almost 80% of the new electrical generating capacity currently being installed in the U.S. makes use of natural gas fired gas turbines (Touchton, 1996). Unfortunately, the combustion process that occurs in these units generates ppm levels of NOX (about a 9:1 molar ratio of NO and NO2) along with the primary combustion products, CO2 and H2O. NOX emissions pose a serious health hazard and also contribute to the formation of acid rain and the environmental damage to forests and aquatic life by acid rain, particularly in the mountains of the Northeast U.S., is well documented (Armor 1992, Hjalmarsson 1990). As a result there has been a strong emphasis on identifying technologies to reduce emissions from these units.
Reducing NOX emissions from combustion sources has been widely studied in the last decade and the work can be divided into two approaches: removing NOX from the effluent after it has been formed (post combustion control) and reducing the amount of NOX that forms during combustion (combustion modification). A number of post combustion methods have been studied, including selective catalytic reduction (SCR). In this method a reducing agent such as ammonia is injected into the waste stream, which is intended to reduce NO and NO2 to nitrogen, even in the presence of percent levels of oxygen.
Although post combustion control methods show promise, they are also expensive. In addition, SCR requires the use of ammonia along with a vanadium-based catalyst; both of these compounds are hazardous and toxic. In addition to be effective, the concentration of ammonia must be continually adjusted to match the NOX concentration in the effluent. If the ammonia concentration is too low, NOX emissions will result; if it is too high, the effluent will contain ammonia. Thus, the system must contain sophisticated monitoring equipment to measure the NOX concentration and the ammonia must be extremely well distributed so that it can react with all of the NOX in the effluent.
The second strategy for reducing NOX emissions is to modify combustion conditions so that the amount of NOX that forms during combustion is reduced. The difficulty with this approach, however, is that NOx formation is a complicated process. During combustion, NOX forms by at least three different mechanisms and it can occur either in the flame zone or out of the flame. NOX formation in the flame zone is rapid and is known as “prompt” NOX. Three mechanisms are responsible for prompt NOX, the Fenimore, the Zeldovich, and the N2O pathways. Briefly, in the Fenimore mechanism a hydrocarbon fragment (HC) reacts with N2 to form HCN, which is then converted to NO. In the Zeldovich mechanism, an oxygen atom reacts with N2 to form NO and an N atom. Finally, in the N2O mechanism, an oxygen atom combines with N2 to form N2O (Schlegel et al. 1994). NOX that forms in the post combustion zone is referred to as “thermal” NOX. In the post combustion zone, only the Zeldovich mechanism contributes to thermal NOX and the process is slower than the Fenimore mechanism.
The primary approach to reducing NOX levels during combustion is to lower the combustion temperature. Because the concentration of NOX does not reach the value that is predicted by thermodynamic equilibrium (approximately 1 mole percent at 2000° C.), we conclude that under combustion conditions NOX formation is limited by kinetics or by the rate at which the NOX formation reactions occur during the combustion process. Under kinetically limited conditions, the rate of NOX formation is a strong function of temperature and thus lowering combustion temperatures even a small amount will significantly reduce NOX concentrations in the exhaust. Probably the best strategy to reduce combustion temperatures is to add excess air prior to combustion. In a turbine, significant amounts of air are already added downstream of the combustor to reduce the temperature to values which the turbine blades can handle, about 1300° C. and therefore rerouting some of this diluent air so that it passes through the combustor is a relatively straightforward modification.
Test results have shown that operating the combustor under lean conditions (i.e. 4% methane) can reduce NOX. Arai and Machida (1991) demonstrated that if combustion is conducted at the lean limit, the combustion temperature is approximately 1900° C. (600° C. lower than that encountered in stoichiometric combustion). This decrease in combustion temperature produced a significant reduction in NOX emissions. In addition, using lean combustion to reduce NOX emissions was the goal of the Department of Energy's Advanced Turbine Systems (ATS) program. As a part of this program several major turbine manufacturers developed high efficiency lean burn combustors, with NOX emissions of less than 10 ppm (Gates 2000, Macri, 2000).
Although significant decreases in NOX emissions have been achieved, further reductions will be required to meet the increasingly restrictive regulations anticipated in the future. However, achieving further reductions in NOX emissions by optimizing lean combustion technology may be difficult because the lean combustors are operating at or very near the lean combustion limits. Thus, additional air cannot be added upstream of the combustor without dropping the fuel concentration below the lean combustion limit, which would cause unacceptable instabilities in the combustion flame. Thus, in order to achieve further reductions in NOX emissions, combustor modifications that allow the addition of more diluent air must be identified.
One approach to operating outside of the lean combustion limits is to install a combustion catalyst in the combustor of the turbine. Because catalytic oxidation does not require a flame, sufficient air can be added to drop the maximum combustion temperature to a level that produces very little NOX. The challenge to this problem is identifying a catalyst that has good activity at low temperature, but that also has excellent thermal stability at high temperature. The catalyst must be able to convert methane at the temperature at which the air exits the compressor (about 350-400° C.) and it must withstand the very high temperatures encountered at the end of the combustor, which could reach 1300° C. Since traditional catalysts sinter and lose essentially all of their activity at temperatures above 800° C., the development of catalysts for catalytic combustion is an active area of research.
Because most catalysts currently available cannot tolerate very high combustion temperatures, one approach is to use a catalyst at the front end of the combustor to initiate the reaction. By limiting the conversion that occurs in this zone to less than 50%, the catalyst temperature can be maintained below 800° C., where many catalysts have adequate thermal stability. The combustion is then completed in a post-catalyst burn out zone, where the temperature can exceed 1300° C. Recent reports (Dalla Betta 1997, Dalla Betta and Rostrup-Nielsen 1999, and Spivey et al. 1994) report that the use of supported palladium catalysts for a catalytic combustor can achieve NOX emissions of between five and ten 10 ppm.
Unfortunately achieving further reductions using this strategy may be difficult. Calculations (illustrated in FIG. 1), which were carried out for a flame at 1500° C., show that about 60% of the total NO is produced as a result of the Fenimore mechanism (Schlegel et al. 1994) which occurs in the flame zone. The authors also carried out experiments in which they varied the ratio of fuel converted over an alumina-supported platinum catalyst to that converted in a post catalyst combustion zone. The total conversion was maintained at 100% and a constant flame temperature was maintained in each series. They found that for a given flame temperature, the measured NOX levels decreased in a linear fashion as the percent of fuel converted over the catalyst increased until the conversion reached about 80%. Between 80 and 100% conversion, the decrease in NO emission was more rapid. These results indicate that to reduce NOX levels below 5 ppm, conversions approaching 100% must be achieved in the presence of the catalyst. Thus, it might be necessary to eliminate the flame completely to produce further reductions in NOX emissions. One way to eliminate the flame would be to carry out the entire combustion process in the presence of a solid catalyst (Fant et al. 2000, Beebe et al. 2000).
An additional problem is associated with the use of traditional palladium catalysts supported on a high surface area material such as alumina. As the temperature increases above 800° C., palladium converts from palladium oxide to palladium metal and as the temperature drops, it reverts back to the oxide form. The combustion rate is different over the two forms of the catalyst and as a result the performance of the combustor can change with load. Designing a combustor that accommodates changing catalyst performance is difficult. It is more desirable to design the combustor based on a constant, well-characterized catalyst performance (Fant et al. 2000). Thus, if palladium is incorporated into a catalyst, it should be done in a fashion that it is retained in a single oxidation state over a wide range of conditions.
Catalytic combustion has the potential to meet future NOX emission standards as long as most or all of the combustion is carried out in the presence of the catalyst. This means that the catalyst must be able to withstand very high temperatures without undergoing deactivation. In addition, if palladium is used, it should be incorporated into the catalyst in such a way that it will not undergo the oxidation-reduction cycle which it does as a supported metal. No catalyst currently produced meets these demanding criteria. Traditional supported metal catalysts sinter and lose essentially all of their activity at these extreme temperatures. In addition, traditional supports are inert and will not inhibit the detrimental tendency of palladium to cycle between the metal and the oxide.
Because catalytic reactions occur on the surface of the catalyst, these materials must have a sufficient surface area to allow the reaction to occur at a reasonable rate. Catalysts typically are composed of metal oxides, which either have catalytic activity of their own or are used to support other active metals, which are dispersed on the support in the form of small crystallites. Iron, cobalt, and manganese are examples of oxides that have activity of their own, while alumina, silica, and titania are considered to be relatively inactive and are used to support more active metals such as platinum or palladium.
Unfortunately, both types of catalysts would not be stable under the extreme temperatures encountered during the combustion of percent levels of methane. At combustion temperatures (approaching 1300° C.), the metal crystallites in the supported catalysts would either agglomerate, which would reduce the active metal surface area or could volatilize, resulting in a permanent loss of the catalytically active metal. In addition, the metal oxide support could sinter causing the pores in the support to collapse, preventing the active metals inside these pores from being catalytically active and further reducing the available metal surface area. Likewise, catalytically active metal oxides such as those of iron, cobalt, and manganese would not be suitable for use under these conditions because they would lose essentially all of their surface area by sintering, rendering them inactive.
However, it has recently been reported that aluminum oxide (Al2O3) can be modified, potentially allowing it to have both the required activity and stability for use as a combustion catalyst. The modification is accomplished by incorporating one or more heteroatoms into the aluminum oxide structure forming a structure known as a hexaaluminate. The heteroatoms give the otherwise inert oxide good activity for methane combustion and also greatly increase its thermal stability. Once incorporated into the lattice, the aluminum oxide forms a hexaaluminate structure, which comprises a series of spinel like blocks that are separated by mirror planes containing the heteroatoms, typically alkali or alkaline earth cations (Arai and Machida 1991, Arai and Machida 1996, Suh et al. 1995, Groppi et al. 1993, Spivey 1994, Wachowski et al. 1994). This material has excellent thermal stability because the crystal growth in the direction normal to the plane is much slower than the rate of growth in the directions parallel to the plane even at temperatures of up to 1500° C. (Arai and Machida 1991). As a result, the crystals do not sinter even at very high temperatures and therefore these materials have excellent potential to be used as combustion catalysts.
Unfortunately, hexaaluminate catalysts are difficult to synthesize using conventional methods. Thus, it is difficult to synthesize catalysts that contain the desired concentrations of substituent metals, making it difficult to study activity and thermal stability on the laboratory scale and also reducing the probability of synthesizing a selected catalyst formulation for a full-scale application. Currently, hexaaluminate materials are prepared either by coprecipitation or by the hydrolysis of alkoxide precursors. In coprecipitation methods, an acidic solution of aluminum nitrate is prepared. Then the desired quantities of the one or two heteroatoms to be incorporated into the hexaaluminate are added to the solution (also as the nitrate salt). At this point, the solution is heated and stirred and the pH is raised at a controlled rate with the addition of base such as ammonium carbonate. The various metals then precipitate as hydroxides or carbonates, the solid is separated by filtration, and then heated to 1300° C.
There are two primary difficulties with this process. First, it is difficult to control the stoichiometry of the hexaaluminate product. Different metals precipitate at different pH's, and it is therefore unlikely that two or more metal oxides that are well-mixed on a micron scale will be obtained. If the two or three compounds are not in very close contact during the heating process, some portions of the alumina may contain too much heteroatom, while other portions may contain too little or none. Moreover, incorporation of two or more heteroatoms, which may be necessary to produce a very active catalyst, would be nearly impossible to control. The second problem is that the average size of the aluminum oxide particles typically is on the order of microns, requiring that the heteroatom travel these distances to incorporate evenly into the aluminum oxide matrix. However, if the migration distance is too great and the aluminum oxide remains at temperature too long without incorporating the heteroatom, then the oxide will convert to the low surface area α (alpha) form before the hexaaluminate structure is formed.
Another route to the production of hexaaluminate compound is the use of metal alkoxides, which are soluble in solvents such as alcohols. Catalysts are prepared by combining the aluminum alkoxide with an alkoxide of the desired cations and then adding water, which results in the formation of a gel containing the metals. Alternatively, salts of the substituent metals can be dissolved in water and then combined during the hydrolysis step (Artizzu-Duart et al. 2000). Although the use of alkoxides has been shown to produce higher surface area compounds relative to those produced by coprecipitation (Spivey 1994, Cinibulk 1995), the alkoxide precursors are expensive. In addition, they are unstable because as mentioned above the addition of water causes hydrolysis to occur and therefore they must be stored or handled in a moisture free environment. Finally, preparation of catalysts using alkoxides requires the use of large quantities of an organic solvent such as ethanol or propanol. Handling large quantities of such solvents can be difficult because they are toxic and flammable.
There has been a strong effort recently to develop catalysts that have good stability at the extreme temperatures encountered in a gas turbine combustor. Catalyst materials typically used in these studies consist of refractory metal oxides such as hexaaluminates and perovskites. In the following sections we discuss recent work on each of these materials.
Hexaaluminate combustion catalysts substituted with a wide variety of heteroatoms including barium, manganese, magnesium, strontium, lanthanum have been prepared and studied as combustion catalysts. For example Jang et al. (1999) conducted tests with coprecipitated hexaaluminates containing barium (BaAl12O19−δ) barium and manganese (BaMn0.5Al11.5O19−δ and BaMnAl11O19−δ), and barium, lanthanum, and manganese (Sr0.8La0.2MnAl11O19−δ). They compared the surface areas and activities of a series catalysts calcined at 1400° C. with the formula Sr1−XLaXMnAl11O19−δ where X=0, 0.2, 0.4, 0.6, 0.8 and 1.0. They reported surface areas ranging from 12.9 to 19.2 m2/g and T10% (temperatures for 10% conversion) from 450° C. to 500° C. (the T1/2 values are about 120° C. higher than the T10%). The catalyst with the highest surface area and most activity was for the case where X=1 (LaMnAl11O19−δ). They also monitored the performance of a coprecipitated hexaaluminate that was maintained at temperature of 600° C. for 100 hours and found that the conversion remained unchanged over this performance period.
Groppi et al. (1993) conducted tests with hexaaluminate-based catalysts composed of Ba-Al-O and Ba-Mn-Al-O, prepared by coprecipitation methods. Although they reported that these catalysts converted up to 40-50% of a methane feed at 600° C., at space velocity of 48,000 h−1, they did not report any results showing that these materials are stable at combustion temperatures. In another study, Groppi et al. (2001) conducted tests with lanthanum-substituted hexaaluminate catalysts prepared by coprecipitation methods. They reported that the presence of magnesium and manganese ions increased the activity of the catalyst, resulting in T1/2 values of between 580 and 620° C. when tested at a feed rate of 54,000 (cc feed/g cat h) which is equivalent to a GHSV of about 25000 h−1 (cc feed/ cc cat h) assuming a catalyst density of about 0.5. Again, this study did not report any data relating to the stability of the catalyst when aged at temperatures expected under combustion conditions.
In U.S. Pat. No. 5,823,761, Euzen et al. (1998) report the use of a staged injector where the second stage catalyst consists of monolithic support and a catalyst that contains cerium, iron and zirconium along with either palladium or platinum. One claim includes the use of a hexaaluminate catalyst. In U.S. Pat. No. 5,830,822 Euzen (1998) reports a thermally stable catalyst with the formula A1−XBYCZAl12−Y−ZO19−δ where A represents either barium or strontium, B is manganese, cobalt, or iron, and C is either magnesium or zinc. In tests at 50,000 h−1, they reported T1/2 values in excess of 650° C. Finally, in U.S. Pat. No. 5,899,679, Euzen et al. (1999) reports a two stage process where the first stage contains platinum or palladium and the second stage catalyst has the formula A1−XBYCZAl12−Y−ZO19−δ, which was described in the previous patent.
Numerous studies have also been conducted on hexaaluminate that have been prepared using alkoxide precursors. For example, Artizzu-Duart et al. (2000) characterized thermal stability of barium-substituted hexaaluminates prepared in this manner. They aged their samples at 1200° C. for 24 hours and observed a significant loss in surface area and lower activity for all samples tested. The catalyst that performed the best, BaFeMnAl10O19, exhibited a small loss in activity evidenced by an increase in T1/2 from 560 to 570° C. However, more importantly, this catalyst lost 27% of its surface area following this aging step (15 m2/g to 11 m2/g). McCarty et al. (1999) also evaluated the stability of lanthanum-substituted hexaaluminates prepared using alkoxide precursors. They conducted a sintering study of a LaAl11O18 material at 1200 and 1400° C., using humid air, which more closely simulates a combustion environment. At 1200° C., the surface area of the catalyst decreased from about 36 m2/g after four hours at temperature to 24 m2/g after 11.5 hours at temperature. This represents a 33% loss in surface area in a period of only 7.5 hours at 1200° C.
Other work using hexaaluminates prepared with alkoxide precursors reports that the catalysts have activity similar to that of catalysts prepared by coprecipitation. For example, Forzotti and Groppi (1999) describe tests performed with BaMn hexaaluminates (BaMnXAl12−XO19). At a space velocity of 48,000 h−1, the T1/2 values for these materials ranged from 640° C. (for X=2) to 760° C. (for X=4), which is similar to the results of Groppi et al. (2001) for barium-substituted hexaaluminates discussed above.
Several groups have investigated hexaaluminate catalysts containing strontium, lanthanum, and manganese prepared by hydrolyzing alkoxide precursors. Woo et al (1998) prepared catalysts with the formula Sr0.8La0.2MnAl11O19 and found that the amount of water used in the hydrolysis step affected the surface area of the material following calcination, with more water causing reduction in surface area. They obtained surface areas ranging from 15 m2/g (less water) to 4 m2/g (more water) following calcination at 1400° C. for 5 hours. Kikuchi et al. (2001) measured the stability of a thin layer of a Sr0.8La0.2MnAl11O19−δ catalyst prepared using metal alkoxides supported on a layer of aluminum titanate. They obtained a relatively high T1/2 of 750° C. when tested at 140,000 h−1 provided an alumina interlayer was present. Without the interlayer, the catalyst was not as active. Finally, Spivey et al. (1994) prepared catalysts of the structure Sr1−XLaXMnAl11O19−δ. They found that T10% values (temperature at which 10% conversion is obtained) of between 450° C. and 550° C. at a space velocity of 53,000 h−1.
Wachowski et al. (1994) prepared samples of hexaaluminates substituted with La, Ce, Pr, Nd and Sm using the alkoxide method. They report that the addition of La had a much greater stabilizing effect on the surface area upon calcination at 1200° C. compared to that of Ce. With La contained in the matrix, surface areas ranged between 50 and 100 m2/g following calcination at 1200° C. On the other hand when Ce was used, the surface areas reported averaged about 10 m2/g. Finally, if no cation was present, the surface area was reported to be about 1 m2/g.
Zarur and Ying (2000) report a variation on the alkoxide method for preparation of a barium hexaaluminate catalyst that claims to produce catalysts in nanoparticles. After dissolving aluminum and barium alkoxide in isooctane, a reverse microemulsion was used to hydrolyze the sample. The barium hexaaluminate had a T1/2 of about 620° C. at a space velocity of 60,000 h−1. In addition, when ceria was added to the material the catalyst was reported to be more active, resulting in a T1/2 of 500° C. In U.S. Pat. No. 6,413,489 (2002), Ying and Zarur report that the addition of manganese and lanthanum and cerium oxide to catalysts prepared with the reverse microemulsion also increase activity, reducing the T1/2 from 620° C. to 530 and 590° C.
In addition to hexaaluminates, other materials including perovskites and aluminate-supported metal oxides also have been tested as combustion catalysts. For example, Jang et al. (1999) reported that a La-Mn hexaaluminate catalyst had much better thermal stability compared to a perovskite. They compared the activity of a hexaaluminate, LaMnAl11O19−δ following calcination at 1400° C. to two cobalt-based perovskites Sr0.25La0.75CoO3, one, which had been calcined at 900° C. and another which had been calcined at 1200° C. The hexaaluminate and the perovskite that had been calcined at 900° C. each had T1/2 values of about 530° C., while the perovskite calcined at 1200° C. was much less active with at T1/2 of 680° C.
Batiot-Dupeyrat et al. (2001) examined the activity of lanthanum-based perovskites for periods of up to 25 h under operating conditions at 900° C. In all cases they reported that the activity decreased continually over the test period and they also reported that following the tests, all samples, except one, had lost a significant fraction of their original surface area. These results show that these materials are stable at 900° C. for periods of much less than 25 hours. Finally, the use of copper oxide supported on a high surface area magnesium aluminate spinel was evaluated as a combustion catalyst (Artizzu et al. 1999). The CuO/MgAl2O4 catalyst was found to undergo a severe loss in activity and surface area following aging at 1200° C. for 12 hours. After aging, the T1/2 of this material increased from 550° C. to approximately 800° C. and the surface area dropped from 44.7 m2/g to 2.2 m2/g. Hexaaluminate catalysts can also be useful for the utilization of our natural gas resources. Natural gas is the most abundant, clean, easily extractable energy source in the world today (Liu et al. 2001, Hickman and Schmidt 1993). The world-wide reserves of this resource, approximately 9000 trillion cubic feet, are large enough to replace the dwindling supplies of petroleum in the 21st century (Periana et al. 1993). Unfortunately it is difficult to utilize this resource economically because most of these natural gas reserves are located in remote areas of the world, far from sites of consumption. In addition, methane (which makes up 90% of natural gas) is a gas with a very low boiling point, −164° C. (Liu et al. 2001, Periana et al 1993). Thus, natural gas must be transported for very long distances in cryogenic tanks. The cost of these fuel tanks and the refrigeration processes needed to liquefy the gas result in very high transportation costs, which are passed on to the user.
Current processes that convert methane to methanol or ethane proceed through a syngas intermediate. In such processes, methane is first converted to syngas by steam reforming: This reaction produces an equilibrium mixture of products and must be run at extremely high temperature to reduce the concentration of CO2. The products of this reaction frequently are run through a secondary reformer to increase the ratio of hydrogen to carbon monoxide. Finally, the mixture of syngas can be used to make either methanol or higher chain hydrocarbons either by way of the methanol synthesis or the Fischer Tropsch reactions shown below: These reactions also have technical challenges. The methanol synthesis reaction is equilibrium-limited so that multiple passes are required. In addition, the Fischer Tropsch reaction is not selective for gasoline type products but produces a range of paraffins.
An attractive alternative is to react excess methane directly with oxygen to form hydrocarbons such as ethane or ethylene by partial oxidation reactions:2 CH4+½O2→C2H6+H2O2 CH4+O2→C2H4+2 H2O
Such processes would eliminate the need for reforming, thereby significantly reducing the cost of converting natural gas to chemicals. In addition, other partial oxidation reactions are also potentially attractive. Economic processes to convert ethane to a chemically valuable intermediate such as ethylene would be very useful.C2H6+½O2→C2H4+H2O
Unfortunately, it is extremely difficult to prevent complete oxidation and the formation of CO2. Thus, a great deal of research has been conducted to identify processes to maximize the selectivity for partial oxidation products. One promising method for increasing selectivity is the recent development of short contact time reactors (SCT). As their name implies, short contact time (SCT) reactors operate at very high space velocities, with only very short periods (milliseconds) in which the reactive gases are in contact with the oxidation catalyst. Operating at very high space velocities is thought to suppress the contribution of gas phase oxidation reactions, which are not selective and always produce CO2 (Feeley et al. 2002). In order to suppress the gas phase reactions, space velocities of up to 10,000,000 h−1 have been employed. (Space velocity is the ratio of feed flow to catalyst volume and has units of h−1). These velocities are anywhere from 10 to 100 times the values used in conventional reactors (Hohn et al. 2002, Feeley et al. 2002). The results obtained with these reactors have been impressive. For example, Feeley et al. (2002) report over 90% selectivity for CO and H2 using a 2:1 mixture CH4 and O2, suggesting that SCT reactors may be the answer to economic methane conversion.
The problem with operating at such high space velocities is that the catalyst must have a very high activity or very little conversion will take place. As a result, the catalysts must be operated at extremely high temperatures, where the reaction kinetics are fast. Temperatures as high as 1200° C. have been reported for SCT reactors in the literature (Hohn et al. 2002). Unfortunately, the catalysts used to date in these systems undergo severe deactivation if they are maintained at these temperatures for long periods of time.
While there has been significant research directed towards the identification of thermally stable oxidation and partial oxidation catalysts, there remains a significant need in the art for such catalysts that exhibit desired levels of thermal stability and exhibit desired activity levels.